The invention relates to an arrangement of attenuating the vibration in a roll assembly of a fiber web machine, in which assembly the roll being rotatably suspended at its end on bearings in bearing housings, and the bearing housings being supported on the frame or the foundation of the machine via viscoelastic intermediate piece or pieces.
With the increasing widths and speeds of paper and board machines the vibration of the rolls is becoming an increasingly severe problem.
At the end of a paper or a board machine, the web is wound to a so-called jumbo roll having the width of the machine. This jumbo roll is unwound and cut in a slitter winder to several strips which are then wound up to a so-called customer roll. Vibration is a problem particularly in two-drum or belt type winders. A vibration problem occurring with two-drum winders arises when the harmonics of rotational speed of the paper roll produced on drums excites the natural frequencies of the drums. The same type of a vibration problem occurs also with the reeling drums of reel-ups.
In general the resonance vibration during the operation of a machine or a device is caused by inadequate damping, in other words inadequate dynamic stiffness at the resonance frequency. The situation is often improved by modifying directly the resonating structure so that its damping is improved. For a general example, a free or a forced viscoelastic layer may be glued on top of a thin vibrating plate. Deformations of the plate then create deformations in the viscoelastic material having a high loss factor, whereby the damping of the eigenmode increases.
However, it is sometimes very difficult or impossible to change the resonating structure so that the damping could be improved. A small diameter paper machine roll resonating at its bending eigenmode can be mentioned as an example. The constrained viscoelastic layer attached to the roll shell does not increase significantly the dynamic stiffness at the lowest bending eigenfrequency because of the relatively high elastic energy of the thick roll shell. This type of arrangement does not induce large enough deformations in the viscoelastic layer due to the smallness of the deformations of the roll shell. Thus, the dynamic stiffness of the roll construction must be influenced some other way.
FI patent no. 94458 discloses a method and an apparatus for controlling the vibrations of paper machine rolls. According to the method, the locations of critical speed areas of the roll are changed during the operation. The critical speed is changed by adjusting the mass and/or the stiffness of the roll, and/or the suspension point of the roll. Amending the stiffness of the bearing support at the ends of the roll is suggested as an alternative. Intermediate pieces of elastic material may be placed between the base plate of housings of the end bearings and the frame. The stiffness of the suspension of the bearing housings can be adjusted by adjusting the force with which the bearing housing presses the intermediate pieces against the frame. This pressing force can be adjusted by a cylinder device or a screw.
JP patent publication no. 3082843 discloses an arrangement for attenuating vibrations of a roll. The drive motor of the roll is elastically attached to the frame. The attachment includes a vibration-proof intermediate piece of rubber between the bottom plate of the securing part of the drive motor and the frame. The securing bolts of the bottom plate extend through the frame plate to a cylinder fixed to the bottom surface of the frame plate where they are secured to a piston in the cylinder. There are rubber sleeves under the heads of the securing bolts; thus the attachment of the bottom plate is floating. In the inner surface of the cylinder there is an extension which limits the movement of the piston upwards in the cylinder. There is a spring between the cylinder top and the top of the piston, and a pressure space with pressurized air as the pressure medium between the bottom surface of the piston and the bottom of the cylinder. The piston is at first pushed pneumatically against the extension of the cylinder inner surface whereby the intermediate rubber pieces and the sleeves are subjected to a minimum pressing force. When the pressure of the compressed air under the piston is decreased the piston is moved downwards by the force of the spring above the piston whereby a greater compressing force is directed to intermediate rubber pieces and the rubber sleeves. Thus, the stiffness of the roll suspension can be regulated by means of the pressure medium under the piston.
These kind of arrangements are quite complex and require considerably sophisticated control system to operate. Thus, in practice they are somewhat prone to have disturbances in operation.
FI patent no. 101283 discloses a method in the winding of a paper web, which aims at avoiding vibration induced by the paper roll being wound-up by regulating the running speed of the winder. The running speed of the winder is adjusted based on the rotational speed of the paper roll being produced so that when the rotational speed of the paper roll being produced approaches the vibration range, the running speed is reduced so that the rotational speed of the paper roll being produced decreases to a range below the lower frequency of the vibration zone. Subsequently, the running speed of the winder is raised so that the rotational speed of the paper roll being produced remains constant until the initial running speed of the winder is reached.
This approach is not optimal for all circumstances and it is possible that occasionally potential capacity is lost due to unnecessary speed reductions.